In The Outsiders, a church catches fire where Ponyboy and Johnny were staying. Why are there kids inside the burning church in the first place?

After killing Bob, a Soc, Johnny and Ponyboy escape to this church out in a pretty remote, mountainous location (which is wonderfully symbolic after Ponyboy's dreams of a simple life in the country) based on Dally's advice. After staying there for a while, Dally comes to check on them and takes them for a quick meal before leaving them again.
Interestingly, it is just before they are all aware of the fire that Johnny has a couple of pertinent revelations. First, he considers turning himself in to the police because he sees that as the best possible outcome for both him and Ponyboy. He also asks whether his (abusive) mother has asked about him, and Dally tells him that she hasn't. Pony realizes that as much as the group of friends support and love Johnny, they are no replacement for the actual love of a mother that Johnny has never had.
When they return the the church, they stop short when they realize that it is on fire. A group of kids has gathered in what seems to be a school picnic, and someone becomes frantic, yelling that some of the kids are missing.
Johnny and Pony leap into action, believing the church is ablaze because of one of their cigarettes. They knock out a window, climb into the church, and get all the kids out. Unfortunately, Johnny is hurt badly in the process.

In The Outsiders, Ponyboy and Johnny are hiding in the abandoned church because Johnny killed a Soc during a fight. Although he did this to keep the Socs from killing Ponyboy, Johnny and Ponyboy are scared they'll be arrested. They go to Dally for help, and he gives them money and tells them where to find an old abandoned church where they can hide.
After Johnny and Ponyboy have been hiding for a few days, Dally comes to check on them. He takes the boys to get food and fills them in on news of their fight with the Socs. On the way back to their hideout, they notice the church is on fire and that a group is gathered outside the church. Asking the first grownup he sees, Ponyboy finds out that the children are there for a school picnic on the mountain. A lady soon remarks that "some of the kids are missing" and shares that she told them not to play in the church. They hear faint yelling from inside, and it is at this time that Ponyboy and Johnny go into the burning church to save the children. Dally soon follows them.